The present invention relates to a voltage control apparatus and method for a power supply and, more particularly, to a voltage control apparatus and method for a power supply such as a solar cell.
In recent years, more people are showing growing interest in global environment and energy problems such as global warming, exhaustion of fossil fuels, radioactive pollutions caused by nuclear power plant failures and radioactive wastes, and the like. Under the circumstances, a solar cell is expected promising as an inexhaustible, clean energy source. However, since the output voltage of a solar cell or the like varies largely depending on the solar radiation amount, temperature, and operation voltage, it is required to adjust the load when viewed from the solar cell so as to always output maximum electric power. For this purpose, a method in which the optimum operation voltage of a solar cell is pre-set, and the actual operation voltage is determined to track the pre-set voltage has been proposed. Also, as disclosed in Japanese Patent Laid-Open No. 62-85312, a so-called hill-climbing method in which the operation voltage or current of a solar cell is varied slightly, and variations in electric power at that time are checked to determine an operation voltage, has also been proposed.
However, as exemplified in FIG. 1, the operation voltage of the solar cell responds quickly to variations in solar radiation amount, and when the solar radiation amount abruptly varies, a voltage control apparatus that uses a conventional feedback control system constituted by a comparator, an integrator, and the like cannot often track the optimum operation voltage. In order to suppress the influence of abrupt variations in solar radiation amount, adjustment for setting, e.g., relatively large parameters such as a proportional gain is performed, or a differentiator is added. However, in this case, the influence of noise may become serious, and so the operation may become unstable.
When optimum operation point tracking control is performed, the operation voltage or current of a solar cell is varied slightly, and variations in electric power at that time are checked. At this time, a wrong operation voltage command may be issued unless a constant operation voltage is set, resulting in operation errors.